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Lecture slides prepared by Dr Lawrie Brown (UNSW@ADFA) for “Data and
o!puter o!!unications"# $%e# by Willia! Stallin&s# 'apter “rotocol
Arc'itecture# *%+# and +nternet,Based Applications"-
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*'is .uote fro! t'e start of Stallin&s D$e ' illustrates a /ey issue w'ic' t'is
c'apter e0plores# providing a context for the detailed material in the following parts of
the text.
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When computers, terminals, and/or other data processing devices exchange
data, the procedures involved can be quite complex. eg. file transfer. There
must be a data path between the two computers. But also need:
1Source to acti2ate co!!unications at' or infor! networ/ of destination
1Source !ust c'ec/ destination is prepared to recei2e
1File transfer application on source !ust c'ec/ destination file !ana&e!ent
syste! will accept and store file for 'is user
13ay need file for!at translation
Instead of implementing the complex logic for this as a single module, the task
is broken up into subtasks, implemented separately. In a protocol architecture,the modules are arranged in a vertical stack, each layer in the stack performs a
related subset of the functions. It relies on the next lower la er to erform more
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Communication is achieved by having the corresponding, or peer, layers in
two systems communicate. The peer layers communicate by means of
formatted blocks of data that obey a set of rules or conventions known as aprotocol. The key features of a protocol are:
• Syntax: Concerns the format of the data blocks
• Semantics: Includes control information for coordination and error handling
• Timing: Includes speed matching and sequencing
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The TCP/IP protocol architecture is a result of protocol research and
development conducted on the experimental packet-switched network,
ARPANET, funded by the Defense Advanced Research Projects Agency(DARPA), and is generally referred to as the TCP/IP protocol suite. This
protocol suite consists of a large collection of protocols that have been issued
as Internet standards by the Internet Activities Board (IAB).
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In general terms, communications can be said to involve three agents:
applications (eg. file transfer), computers (eg. PCs & servers), and networks.
These applications, and others, execute on computers that can often supportmultiple simultaneous applications. Computers are connected to networks, and
the data to be exchanged are transferred by the network from one computer to
another. Thus, data transfer involves first getting the data to the computer in
which the application resides and then getting the data to the intended
application within the computer. Can think of partitioning these tasks into 3
layers as shown.
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*%+ doesn4t 'a2e an “official" layer !odel (5 it predates t'e 6S+ 7eference
3odel we4ll introduce later)# but it does 'a2e a “wor/in&" layer !odel# as
s'own-
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The physical layer covers the physical interface between a data transmission
device (e.g., workstation, computer) and a transmission medium or network.
This layer is concerned with specifying the characteristics of the transmissionmedium, the nature of the signals, the data rate, and related matters.
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The network access layer is concerned with the exchange of data between an
end system (server, workstation, etc.) and the network to which it is attached.
The sending computer must provide the network with the address of thedestination computer, so that the network may route the data to the appropriate
destination. The sending computer may wish to invoke certain services, such as
priority, that might be provided by the network. The specific software used at
this layer depends on the type of network to be used; different standards have
been developed for circuit switching, packet switching (e.g., frame relay),
LANs (e.g., Ethernet), and others. Thus it makes sense to separate those
functions having to do with network access into a separate layer.
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The internet layer provides procedures used to allow data to traverse multiple
interconnected networks, to provide communications between devices are
attached to different networks. The Internet Protocol (IP) is used at this layer toprovide the routing function across multiple networks. This protocol is
implemented not only in the end systems but also in routers. A router is a
processor that connects two networks and whose primary function is to relay
data from one network to the other on its route from the source to the
destination end system.
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The host-to-host layer, or transport layer, collects mechanisms in a common
layer shared by all applications to provide reliable delivery of data. Regardless
of the nature of the applications, there is usually a requirement that data beexchanged reliably, ensuring that all of the data arrives at the destination
application and that the data arrives in the same order in which they were sent.
These mechanisms for providing reliability are essentially independent of the
nature of the applications. The Transmission Control Protocol (TCP) is the
most commonly used protocol to provide this functionality.
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Finally, the application layer contains the logic needed to support the various
user applications. For each different type of application, such as file transfer, a
separate module is needed that is peculiar to that application.
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Stallin&s D$e Figure 2.1 indicates how these protocols are configured for
communications. To make clear that the total communications facility may
consist of multiple networks, the constituent networks are usually referred to assubnetworks. Some sort of network access protocol, such as the Ethernet
logic, is used to connect a computer to a subnetwork. This protocol enables the
host to send data across the subnetwork to another host or, if the target host is
on another subnetwork, to a router that will forward the data. IP is
implemented in all of the end systems and the routers. It acts as a relay to move
a block of data from one host, through one or more routers, to another host.
TCP is implemented only in the end systems; it keeps track of the blocks of
data to assure that all are delivered reliably to the appropriate application.
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For successful communication, every entity in the overall system must have a
unique address. Actually, two levels of addressing are needed. Each host on a
subnetwork must have a unique global internet address; this allows the data tobe delivered to the proper host. Each process with a host must have an address
that is unique within the host; this allows the host-to-host protocol (TCP) to
deliver data to the proper process. These latter addresses are known as ports.
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onsider a simple operation where a process on host A, wishes to send a
message to another process on host B. The process at A hands the message
down to TCP with instructions to send it to host B. TCP hands the messagedown to IP with instructions to send it to host B. Note that IP need not be told
the identity of the destination port. Next, IP hands the message down to the
network access layer (e.g., Ethernet logic) with instructions to send it to router
J (the first hop on the way to B).
To control this operation, control information as well as user
data must be transmitted, as suggested in Stallings DCC8e Figure 2.2. The
sending process generates a block of data and passes this to TCP. TCP may
break this block into smaller pieces to make it more manageable. To each ofthese pieces, TCP appends control information known as the TCP header,
forming a TCP segment.
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For most applications running as part of the TCP/IP protocol architecture, the
transport layer protocol is TCP. TCP provides a reliable connection for the
transfer of data between applications. A connection is simply a temporarylogical association between two entities in different systems. A logical
connection refers to a given pair of port values. For the duration of the
connection each entity keeps track of TCP segments coming and going to the
other entity, in order to regulate the flow of segments and to recover from lost
or damaged segments.
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* se&!ents include a 'eader- Stallings DCC8e Figure 2.3a shows the headerformat for TCP, which is a minimum of 20 octets, or 160 bits. The Source Port
and Destination Port fields identify the applications at the source anddestination systems that are using this connection. The Sequence Number,Acknowledgment Number, and Window fields provide flow control and errorcontrol. The checksum is a 16-bit frame check sequence used to detect errors inthe TCP segment. Chapter 20 provides more details.
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In addition to TCP, there is one other transport-level protocol that is in
common use as part of the TCP/IP protocol suite: the User Datagram Protocol
(UDP). UDP does not guarantee delivery, preservation of sequence, orprotection against duplication. UDP enables a procedure to send messages to
other procedures with a minimum of protocol mechanism. Some transaction-
oriented applications make use of UDP; eg SNMP (Simple Network
Management Protocol). Because it is connectionless, UDP has very little to do.
Essentially, it adds a port addressing capability to IP.
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Because it is connectionless, UDP has very little to do. just adding a port
addressing capability to IP. This is best seen by examining the UDP header,
shown in Stallings DCC8e Figure 2.3b. The UDP header also includes achecksum to verify that no error occurs in the data; the use of the checksum is
optional.
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For decades, the keystone of the TCP/IP protocol architecture has been IP.
Stallings DCC8e Figure 2.4a shows the IP header format, which is a minimum
of 20 octets, or 160 bits. The header, together with the segment from thetransport layer, forms an IP-level PDU referred to as an IP datagram or an IP
packet. The header includes 32-bit source and destination addresses. The
Header Checksum field is used to detect errors in the header to avoid
misdelivery. The Protocol field indicates which higher-layer protocol is using
IP. The ID, Flags, and Fragment Offset fields are used in the fragmentation and
reassembly process. Chapter 18 provides more detail.
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In 1995, the Internet Engineering Task Force (IETF), which develops protocol
standards for the Internet, issued a specification for a next-generation IP,
known then as IPng. This specification was turned into a standard in 1996known as IPv6. IPv6 provides a number of functional enhancements over the
existing IP, designed to accommodate the higher speeds of today's networks
and the mix of data streams, including graphic and video, that are becoming
more prevalent. But the driving force behind the development of the new
protocol was the need for more addresses. The current IP uses a 32-bit address
to specify a source or destination. With the explosive growth of the Internet
and of private networks attached to the Internet, this address length became
insufficient to accommodate all systems needing addresses. As Stallings
DCC8e Figure 2.4b shows, IPv6 includes 128-bit source and destinationaddress fields. Ultimately, all installations using TCP/IP are expected to
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A number of applications have been standardized to operate on top of TCP. We
mention three of the most common here.
The Simple Mail Transfer Protocol (SMTP) provides a basic
electronic mail transport facility for transferring messages among separate
hosts. The SMTP protocol does not specify the way in which messages are to
be created; some local editing or native electronic mail facility is required. The
target SMTP module will store the incoming message in a user's mailbox.
The File Transfer Protocol (FTP) is used to send files from
one system to another under user command. Both text and binary files are
accommodated. FTP sets up a TCP connection to the target system for the
exchange of control messages. Once a file transfer is approved, a second TCPdata connection is set up for the data transfer, without the overhead of any
headers or control information at the a lication level. When the transfer is
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Each layer in the TCP/IP protocol suite interacts with its immediate adjacent
layers. This use of each individual layer is not required by the architecture. As
Stallings DCC8e Figure 2.5 suggests, it is possible to develop applications thatdirectly invoke the services of any one of the layers. Most applications require
a reliable end-to-end protocol and thus make use of TCP. Some special-
purpose applications do not need the services of TCP. Some of these
applications, such as the Simple Network Management Protocol (SNMP), use
an alternative end-to-end protocol known as the User Datagram Protocol
(UDP); others may make use of IP directly. Applications that do not involve
internetworking and that do not need TCP have been developed to invoke the
network access layer directly.
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The Open Systems Interconnection (OSI) reference model was developed by
the International Organization for Standardization (ISO - which is not an
acrony! but a word# deri2ed fro! t'e 8ree/ isos# !eanin& e.ual) as a modelfor a computer protocol architecture and as a framework for developing
protocol standards. The OSI model consists of seven layers. The designers of
OSI assumed that this model and the protocols developed within this model
would come to dominate computer communications, eventually replacing
proprietary protocol implementations and rival multivendor models such as
TCP/IP. This has not happened. Although many useful protocols have been
developed in the context of OSI, the overall seven-layer model has not
flourished. Instead, the TCP/IP architecture has come to dominate.
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Stallin&s D$e Figure 2.6 illustrates the OSI model and provides a brief
definition of the functions performed at each layer. The intent of the OSI
model is that protocols be developed to perform the functions of each layer.
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There are a number of reasons why the TCP/IP architecture has come to
dominate. Perhaps the most important is that the key TCP/IP protocols were
mature and well tested at a time when similar OSI protocols were in thedevelopment stage. When businesses began to recognize the need for
interoperability across networks, only TCP/IP was available and ready to go.
Another reason is that the OSI model is unnecessarily complex, with seven
layers to accomplish what TCP/IP does with fewer layers. Stallin&s D$e
Figure 2.7 illustrates the layers of the TCP/IP and OSI architectures, showing
roughly the correspondence in functionality between the two.
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The principal motivation for the development of the OSI model was to provide
a framework for standardization. Within the model, one or more protocol
standards can be developed at each layer. The model defines in general termsthe functions to be performed at that layer and facilitates the standards-making
process in two ways by allowing standards to be developed independently and
simultaneously for each layer, and because changes in standards in one layer
need not affect already existing software in another layer.
Stallin&s D$e Figure 2.8 illustrates the use of the OSI model
as such a framework. The overall communications function is decomposed into
seven distinct layers, making the interfaces between modules as simple as
possible. In addition, the design principle of information hiding is used: Lower
layers are concerned with greater levels of detail; upper layers are independent
of these details. Each layer provides services to the next higher layer and
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Stallin&s D$e Figure 2.9 shows more specifically the nature of the
standardization required at each layer. Three elements are key:
• Protocol specification: Two entities at the same layer in different systemscooperate and interact by means of a protocol. Because two different open
systems are involved, the protocol must be specified precisely. This includes
the format of the protocol data units exchanged, the semantics of all fields, and
the allowable sequence of PDUs.
• Service definition: In addition to the protocol or protocols that operate at a
given layer, standards are needed for the services that each layer provides to
the next higher layer. Typically, the definition of services is equivalent to a
functional description that defines what services are provided, but not how theservices are to be provided.
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The services between adjacent layers in the OSI architecture are expressed in
terms of primitives and parameters. A primitive specifies the function to be
performed, and the parameters are used to pass data and controlinformation. The actual form of a primitive is implementation dependent.
An example is a procedure call. The layout of Stallin&s D$e Figure
2.10a suggests the time ordering of these events. For example, consider the
transfer of data from an ( N ) entity to a peer ( N ) entity in another system.
The following steps occur:
1. The source ( N ) entity invokes its ( N –1) entity with a request primitive
including needed parameters,such as the data to be transmitted and the
destination address.
2. The source ( N –1) entity prepares an ( N –1) PDU to be sent to its peer ( N –
1 entit .
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Stallin&s D$e Table 2.1 defines the four types of primitives are used in
standards to define the interaction between adjacent layers in the architecture
(X.210)
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The Internet, until recently, has been dominated by information retrieval
applications, e-mail, and file transfer, plus Web interfaces that emphasized text
and images. Increasingly, the Internet is being used for multimedia applicationsthat involve massive amounts of data for visualization and support of real-time
interactivity. Streaming audio and video are perhaps the best known of such
applications.
Although traditionally the term multimedia has connoted the
simultaneous use of multiple media types (e.g., video annotation of a text
document), the term has also come to refer to applications that require real-
time processing or communication of video or audio alone. Thus, voice over IP
(VoIP), streaming audio, and streaming video are considered multimedia
applications even though each involves a single media type.
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Traffic on a network or internet can be divided into two broad categories:
elastic and inelastic. Elastic traffic can
adjust, over wide ranges, to changes in delay and throughput across an internetand still meet the needs of its applications. This is the traditional type of traffic
supported on TCP/IP-based internets and is the type of traffic for which
internets were designed. Elastic applications include common Internet-based
applications, such as file transfer, electronic mail, remote logon, network
management, and Web access. But there are differences among the
requirements of these applications.
Inelastic traffic does not easily adapt, if at all, to changes in
delay and throughput across an internet. The prime example is real-time traffic,
such as voice and video. The requirements for inelastic traffic may include the
following: minimum throughput may be required, may be delay-sensitive, may
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Stallin&s D$e Figure 2.11 looks at multimedia from the perspective of three
different dimensions: type of media, applications, and the technology required
to support the applications. Consider the list of technologies relevant to thesupport of multimedia applications. As can be seen, a wide range is involved.
The lowest four items on the list are beyond the scope of this book. The other
items represent only a partial list of communications and networking
technologies for multimedia. These technologies and others are explored
throughout the book.
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Stallin&s D$e 'apter su!!ary-
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